Modern cellular digital mobile communications networks have revolutionized mobile communication since the beginning of the 1990s. They have almost completely replaced the existing analog mobile communications networks. Modern standards (in particular GSM, UMTS, CDMA) facilitated a drop in infrastructure and terminal device prices, which led to competition among the telephony providers and to lower charges.
The competition among the companies manufacturing the infrastructure was also promoted as a result of the fact that communication between the network elements was standardized, the interface descriptions and protocols were laid open and consequently mobile communications networks conforming to the aforementioned standards could be built by putting together network elements supplied by any manufacturers. A feature common to all mobile communications networks is that they consist of elements for providing radio coverage (antennas, base stations (in the case of GSM, called “BTS”—Base Transceiver Station), control units (in the case of GSM, called “BSC”—Base Station Controller) and of elements from switching technology (in the case of GSM, called “MSC”—Mobile Switching Center). In addition there also exist various other network elements for accomplishing diverse tasks such as providing a connection to the public telephone system (PSTN), user management and the like.
In the development of these standards attention was focused on providing coverage for the urban and rural area, with cell sizes and interfaces being geared to distances and user densities typical therefor.
If there now exists the desire for mobile communication in areas cut off from conventional infrastructure, such as e.g. remote islands, settlements, aircraft and ships, a possible solution is to set up a complete mobile communications system at said locations and to attach them to the public telephone system by way of, for example, telephone cables, directional radio relay links or satellite radio links. This is associated with very high costs, however. In the case of the use of such a system in an aircraft, the considerable weight of said equipment also constitutes a problem.
If, instead, only the absolutely necessary network elements (terminal devices, base station) are brought to these remote locations and the connection to the rest of the mobile communications system is routed via a transmission system, then the signaling efforts between the mobile terminal device and the mobile communications network that are necessary for registering with the network, but also for setting up a connection, sending an SMS (Short Message Service) or a location update, can fail if said transmission system is subject to a long delay time (e.g. when a satellite radio link is used).
Said registration procedures are based on the fact that messages which contain a request reference are sent by the mobile terminal device to the mobile communications network. The mobile communications network thereupon sends corresponding reply messages to the requesting mobile terminal device. The request references previously transmitted by the mobile terminal device are contained in said messages and thereby enable the mobile terminal device to assign the reply message uniquely. However, the request references transmitted by the mobile terminal device only remain valid for a certain period of time. If a reply message is returned to the mobile terminal device after this time has elapsed (e.g. due to time delays inside the mobile communications network), the reply message is discarded as invalid and the mobile terminal device starts once again to perform a registration procedure. If the time delay continues to be present during this or further registration attempts, the mobile terminal device will not be able to register with said mobile communications network.
Known solutions to this problem are on the one hand careful optimization of the network configuration (in the GSM example, the setting of the so-called cell parameters “max Retrans” and “Tx Integer”), though this too can only provide a remedy up to a maximum signal propagation delay, and the configuring of the mobile communications network in such a way that all the network elements involved in the registration procedure are set up close to the requesting mobile terminal device and only a further interface which can no longer affect the registration procedure (in the GSM example, the so-called “A” interface) is routed via the transmission system with a long time delay. The latter solution involves the already cited disadvantages in terms of higher costs and greater weight.
The signaling of the registration procedure in accordance with the standard “GSM TS 04.08 chapter 3.3.1.1.2 (Version 7.21.0 Release 1998)” shall be described below by way of example. The algorithm of said registration procedure is based on the repeated transmission of signaling requests CHAN RQD on what is termed the random access channel RACH in the direction of the BTS. In this case each CHAN RQD message is provided with a “request reference (Ref.1)” by the mobile terminal device (MS). The mobile communications network (or the base station controller BSC) responds to said message by sending CHAN ACT with the setup of the future signaling channel and thereupon sends what is termed an “immediate assignment message” IMM ASSIGNMENT to the mobile terminal device on the access grant channel AGCH. Said IMM ASSIGNMENT message contains the Ref.1 used by the mobile terminal device and can therefore be assigned by the MS. Since the transmission of the CHAN RQD message is not coordinated with other mobile terminal devices likewise indicating a signaling request, said message can get lost on the air interface. The mobile terminal device therefore repeats the transmission of the CHAN RQD message after a specified period of time (which is determined as a function of the cell configuration S and the Tx-Integer value broadcast by the BTS). For each retransmission the mobile terminal device uses a new reference (Ref n)—and in the process notes only the last three references (Ref n−2, Ref.n−1, Ref.n). Following the transmission of the last repetition of the CHAN RQD message (determined by the Max Retrans value broadcast by the BTS) the mobile terminal device starts the timer T3126 and waits for the arrival of an IMM ASSIGNMENT message matching the last three transmissions. If this timer times out, the mobile terminal device aborts the assignment procedure with the network. The consequence is that a location registration of the mobile, a call setup or the sending of an SMS will fail.